How a multipotent cell becomes committed to a particular differentiation pathway is not known. The long term goal of this research is to understand the controls governing the proliferation and differentiation behavior of a multipotent stem cell, the interstitial cell of hydra. At this stage, the approach is to elucidate the cell-cell interactions regulating each of the several differentiations the interstitial cell can undergo. It is known that the interstitial cell population contains multipotent stem cells and cells committed to a particular differentiation. There is increasing evidence that for this system, and every other system with a multipotent cell, there are cells with intermediate differentiation capacities. Thus, any cell-cell interaction, or cell-molecule interaction thought to affect the multipotent cell, may yield ambiguous results as it may affect a derivative cell instead. Therefore to accurately work out the controls effecting the multipotent cell, a precise knowledge of the complexity of the population in terms of number of cell types with different differentiation capabilities and of the cell lineage relationships among them is necessary. The goals of this proposal are to do that for the interstitial cell system of hydra with the following approach: (1) Monoclonal antibodies that bind specifically to I-cells, to subsets of I-cells, to subsets of I-cells and differentiation intermediates and/or products will be sought. (2) Subsets of I-cells, each defined by a monoclonal antibody, will be examined in pairs to determine their relatedness. This will lead to a map of nearest neighbors which will tentatively be a map of the cell lineage relationships. (3) Using the defining antibody, cells of each subset will be isolated and their differentiation and proliferation capacities examined. This information will be used to validate the relationships described in the map. An understanding of the cellular interactions controlling the differentiation of multipotent cells in basic to an understanding of some classes of defects that occur during human development.